Conventionally, study has been conducted on a multi-mode fiber, and many methods have been proposed for solving a transmission capacity limitation caused by a mode group delay difference. One of the proposed methods is that the mode groups are separated for electrically equalizing the group delay difference (Non-Patent Document 1). After that, based on the fact that the mode propagation angle corresponds approximately to the mode order in a step-index fiber, angular division multiplexing was proposed (Non-Patent Document 2). In addition, in 2006, mode group diversity multiplexing was proposed in which almost the same concept is applied to a graded-index fiber (Non-Patent Document 3). In all fibers described in those documents, each eigen-mode is not used as an independent transmission channel represented in the orthogonal function system of field profile but is used as a mode group for multiplexing/demultiplexing using propagation angle differences, with the result that the transmission capacity is smaller than that of a single-mode fiber. That is, the technologies described in those documents are not those for increasing the transmission capacity to the maximum.
Non-Patent Document 4 describes mode division multiplexing in which each of the modes of a multimode fiber is made to correspond to a transmission channel.
The technology described in Non-Patent Document 5 is known as a technology for implementing a conventional uncoupled multi-core fiber of homogeneous cores using a photonic crystal fiber.
Non-Patent Document 6 discloses a technology for reducing the crosstalk between two cores, which have different propagation constants, to a desired value or lower.